P6 Duration Types Explained
Last reviewed 16 September 20266 min read
Fixed Duration & Units, Fixed Units/Time and the other two — what moves when you edit the third number
🟩 In one line: Every resourced activity holds three numbers tied by one equation — Units = Duration × Units/Time — and the duration type decides which number P6 protects and which it recalculates when you edit another; Fixed Duration & Units is the planner's default because it keeps the duration and the budget where you put them.
👤 Who this is for: J/M. Prerequisites: setting-up-a-p6-project, crew-and-manhour-formulas. Companion to p6-activity-types-explained and percent-complete-types.
First, let's be honest about why this page exists
Duration type does nothing at all until an activity has a resource on it. Then it does one thing, silently: when you change a duration, a budget or a rate, it changes one of the other two. A planner who loads 1,920 manhours on a 24-day activity and then shortens it to 20 days will either see the budget drop to 1,600 hours or the crew size rise — and which one depends on a setting nobody looked at.
The wrong type is the commonest reason a resource-loaded schedule's manhour total drifts between updates without anyone touching the budgets.
🟨 The standard — what "good" looks like
| Source | What it asks for (paraphrased) | Consequence |
|---|---|---|
| AACE RP 38R-06 | Basis records how durations were derived and how resources relate to them | Duration type stated in the SBM tool-settings section |
| GAO Schedule Assessment Guide, BP 3 | Resources assigned; resource assumptions realistic and documented | Budget must not change as a side effect of a duration edit |
| PMI Practice Standard for Scheduling, 2nd ed. | Duration estimating and resource assignment methods consistent across the model | One type for all production work |
| DCMA 14-Point, Metric 10 (Resources) | Incomplete tasks carry resources or cost | Type matters on every loaded activity |
| Convention (crew-and-manhour-formulas) | Manhours = Qty × unit rate; Duration = Manhours ÷ (crew × hours/day) | The planner sets duration and units; units/time is the derived crew — so protect the first two |
🟢 Rule: Fixed Duration & Units on every production activity; the crew size (units/time) is the number P6 is allowed to recalculate; the other two types are used only by exception and recorded in the SBM.
How it actually works
1. The four types and what each recalculates
| Duration type | Protected | When you edit Duration → | When you edit Units → | When you edit Units/Time → |
|---|---|---|---|---|
| Fixed Duration & Units | Duration, Units | Units/Time changes | Units/Time changes | Units change |
| Fixed Duration & Units/Time | Duration, rate | Units change | Units/Time changes | Units change |
| Fixed Units | Units (budget) | Units/Time changes | Duration changes | Duration changes |
| Fixed Units/Time | Rate (crew) | Units change | Duration changes | Duration changes |
Read the first row: shorten a Fixed Duration & Units activity from 24 to 20 days and the budget of 1,920 hours stays; P6 shows the crew going from 8 to 9.6 per day. That is the honest picture — a shorter duration needs more people — and it is the one you want to see and then decide on.
2. When the type actually fires
| Situation | Does duration type act? |
|---|---|
| Activity with no resource assignment | No — durations are just typed numbers |
| Editing duration / units / units-per-time on a resourced activity | Yes |
| Adding or removing a resource | Depends on Edit → User Preferences → Calculations → Resource Assignments ("Preserve units, duration and units/time" vs "Recalculate") |
| Resource levelling | Yes — Fixed Units/Time activities stretch; Fixed Duration ones do not |
| Statusing (actual units, remaining duration) | Yes — Fixed Units/Time will re-forecast remaining duration from remaining units ÷ rate |
| LOE activities | Duration comes from linked dates; units spread over it regardless |
3. Choosing by exception
| Use | Type | Why |
|---|---|---|
| All production work, quantity- or labour-loaded | Fixed Duration & Units | Planner owns duration and budget |
| Supervision, preliminaries (LOE) | Fixed Duration & Units/Time | A fixed staff level spread across whatever span results |
| A genuinely rate-driven activity where the crew is fixed by physical limits — one paving train, one TBM, one shotcrete rig | Fixed Units/Time | Duration should stretch when quantity changes |
| Never on production work | Fixed Units | Duration silently changes when someone edits the crew |
4. Setting it
- Project default: Enterprise → Projects → Details → Defaults tab → Duration Type.
- Per activity: Activity Details → General tab → Duration Type.
- Bulk correction: Global Change (Duration Type = Fixed Duration & Units where Activity Type = Task Dependent), run on a copy first, Schedule Comparison after to prove zero date change.
- SBM tool-settings table lists the default and every exception by filter.
5. The check on a received file
Group by Duration Type. Anything other than Fixed Duration & Units on Task Dependent production work gets a question in the reconciliation table. Fixed Units/Time is the one to look hardest at: every time the subcontractor statused actual hours, P6 re-forecast the remaining duration from the rate — which is exactly the automatic forecasting that forecasting-honestly forbids.
🟥 Where people go wrong
- Fixed Units/Time inherited from a template. Every duration edit changes the budget; every actual-units entry changes the remaining duration. The manhour total drifts 3–5% between updates and nobody can say why.
- Changing type after actuals exist. P6 recalculates using the new rule on the remaining portion; budgets and rates jump on that update. Set it at project set-up; change in a baseline draft only.
- Assuming duration type controls unresourced durations. It doesn't. If your unloaded schedule changed durations, look at calendars and hours-per-day, not here.
- Forgetting the User Preference on adding resources. With "Recalculate units, duration and units/time" selected, adding a second resource can halve the duration. Set to Preserve, and record it.
- One type for LOE and production alike. Supervision on Fixed Duration & Units carries a fixed 12,000-hour budget whether the job takes 20 months or 26. Fixed Duration & Units/Time lets the staff cost follow the span, which is what actually happens.
⚖️ When you're challenged
"I shortened the activity and the budget didn't change. Isn't that wrong?" It's right. The hours to lay 640 m² of blockwork don't fall because we want it done faster — the crew per day goes up, and that's what P6 now shows. If we want the budget to fall we need a method change, not a shorter bar.
"Set everything to Fixed Units/Time so the schedule updates itself from actual hours." That gives a remaining duration from hours spent, not work done. Hours spent tell you about the crew; quantity installed tells you about the finish date. Remaining durations come from the quantity sheet and achieved rate — the type stays Fixed Duration & Units.
📄 Related pages
- Crew and Manhour Formulas — the equation behind the three numbers
- P6 Activity Types Explained — the first of the three activity settings
- Percent Complete Types — the third
- Resource Loading and Histograms — where the type starts to matter
- Resource Levelling — how each type behaves under levelling
- Forecasting honestly — why remaining duration is never rate-derived
- Setting Up a P6 Project — the project default
✏️ Worked example
Level 5 blockwork on a Dubai mid-rise: 640 m², 3.0 mh/m² → 1,920 mh; crew 8 masons × 10 h → 80 mh/day → 24 days. The planner is asked to show a 20-day duration.
| Duration type | After changing 24 → 20 days | What it means |
|---|---|---|
| Fixed Duration & Units | Budget 1,920 mh; units/time 96 mh/day (9.6 masons) | Truthful: 2 more masons needed; take it to the sub |
| Fixed Duration & Units/Time | Budget 1,600 mh; 80 mh/day | 320 mh vanished from the estimate |
| Fixed Units | Budget 1,920 mh; 96 mh/day | Same as row 1 for this edit — but editing the crew would have changed the duration instead |
| Fixed Units/Time | Budget 1,600 mh; 80 mh/day | Same loss as row 2 |
The planner kept Fixed Duration & Units, wrote "Duration Basis: 640 m² ÷ (9.6 masons × 10 h ÷ 3.0 mh/m²); crew increase to 10 confirmed by sub, letter S-044" in the UDF, and set the crew to 10 for a 19-day duration rounded up to 20.
📖 References
- AACE International RP 38R-06, Documenting the Schedule Basis
- GAO Schedule Assessment Guide (GAO-16-89G), Best Practice 3
- PMI Practice Standard for Scheduling, 2nd ed.
- DCMA 14-Point Schedule Assessment, Metric 10
- Oracle Primavera P6 Professional User Guide — Duration Types; User Preferences → Calculations → Resource Assignments (field labels as in P6 Professional v20; earlier versions differ)
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